Such a system and vessel are known from WO 2013/125951, filed in the name of the applicant. In this publication it is described that efficient air lubrication of a flat bottom of a vessel is achieved by providing a relatively small-sized open cavity and injecting air into the cavity at about hydrostatic pressure, such that a substantially flat water-air interface is formed at the height of the bottom. At this interface, air is mixed into the water due to the Kelvin Helmholtz mixing effect, and a stream of air bubbles escapes from the rear of the cavity. Such cavities were found to provide a stable and efficient manner of providing a layer of bubbles along the bottom, reducing frictional drag such that the energy gain as a result of reduced friction during propulsion by far outweighs the extra energy required for injecting air at hydrostatic pressures into the cavity.
In order to facilitate emptying of the cavity during start up, a number of curved wave deflectors is described that extend transversely across the cavity. The wave deflectors reduce turbulence inside the cavity and cause air to be retained inside the cavity for a longer period of time such that compressors of reduced capacity for less powerful air injection during start up are required.
In U.S. Pat. No. 6,145,459 an air lubrication system is described in which air is injected along the hull at an angle towards the stern via a slit connected to a cavity containing compressed air on the inside surface of the hull. A wire is placed upstream of the exit point of the slit in the hull to cause turbulence that breaks up the air volume hence forming small bubbles. The known system has as a disadvantage that air injection requires relatively high pressures and is relatively inefficient in view of the power required for lubrication versus reduction in frictional drag. Furthermore, the wire on the outside of the hull is relatively vulnerable, produces additional drag and may form a point of attachment for dirt, shellfish or seaweed.
JP 2002-2582 A describes an air cavity vessel having a cavity and having a special upstream wedge projecting below bottom level for creating an under pressure inside the cavity. Air is supplied to the cavity without the need for a compressor in view of the created under pressure. Inside the cavity, a lower front part results in an uneven and turbulent air-water interface causing mixing of water and air. Small-size bubbles having a relatively low internal pressure exit at the rear of the cavity, the bubble size decreasing due the water pressure when the bubbles travel along the bottom of the hull. The known lubrication system is relatively inefficient as it operates at non-controlled under-pressures created inside the cavity, and fails to form a flat air-water interface that is level with the flat bottom, allowing mixing of air and water by the Kelvin Helmholtz effect along said whole interface and unrestricted exit from the cavity along the interface onto the bottom in a well-defined and undisturbed boundary layer.
In case the vessel sails in shallow waters, the air cavities and/or air ducts supplying air to the cavities may become filled and blocked by silt or soil or other materials originating from the sea bed.
For maintenance purposes the cavities should be easily inspected when the vessel is in dry dock. Also, access to the cavity interior for maintenance purposes should be facilitated.
It is found that marine growth on the vessel hull, such as barnacles or other crustaceous growth on the vessel bottom, or weed or algae on the vessel sides, highly increases the water resistance.
It is therefore an object of the present invention to provide a method of reducing water resistance. It is in particular an object of the invention provide a method for reducing marine growth on the hull.
It is a further object of the invention to provide a system for air lubrication that has an air supply duct which can be easily cleaned from silt or soil, or other blocking substances entering into the air outlet duct.
Other objects are to provide an air cavity system that has improved efficiency during start up and that shows reduced instabilities inside the cavity. It is a further object to provide a system and vessel providing reduced water disturbance in case no air is included in the cavity and a stable air-water interface inside the cavity during roll motions.
The invention aims to provide an improved distribution of the cavities along the bottom for increased air lubrication.
Furthermore, the invention strives to provide improved control and safety of the air lubrication system.
It is another object of the invention to provide an air lubrication system having improved airflow characteristics.